This invention relates generally to wireless communication systems.
Wireless communication has many applications in consumer and business markets. Among the many applications are: mobile wireless, fixed wireless, unlicensed Federal Communications Commission (FCC) wireless, local area network (LAN), cordless telephony, personal base station, telemetry, and others. Generally, each of these applications utilizes unique and frequently incompatible modulation techniques and protocols.
Wireless communication devices, such as cellular handsets, typically need to acquire certain cell specific information and characteristics before negotiating a service with a base station. For this purpose, each base station transmits certain cell specific information necessary for a user to acquire services such as paging or cellular telephony from the base station. For example, in CDMA systems, the cell specific information is contained in pilot and/or synchronization channels. The pilot and/or synchronization channels are spread and scrambled with cell specific pseudo-random noise or gold code sequences, which form the basis for frame, slot, and bit timing synchronization for a handset.
Multiple users are typically in communication with a single base station. Although such users operate on the same frequency at the same time, various techniques allow multiple users to be distinguished from one another. In CDMA systems, each handset user is assigned a different orthogonal code that is used to spread the data transmitted from the base station so as to distinguish it from the data transmitted to other handset users.
FIG. 1 illustrates a prior art communication device 100. The communication device 100 includes an antenna 102, a front-end processor 104, a base band processor 106, a microprocessor/controller 108, and a bus for interconnecting the front-end processor 104, the base based processor, and a microprocessor/controller 108. The microprocessor 108 supports the exchange of data and/or instructions to the other components of the communication device 100. The base band processor 106 is coupled to the front-end processor 104 to receive and transmit data. The communication device 100 may be a mobile handset, a test platform, an embedded modem, a base station or other communication devices in other code-dependent applications.
The front-end processor 104 is coupled to the antenna 102 to receive data. The front-end processor 104 includes components and performs functions that are known to those skilled in the art. These components are not shown in the front-end processor 104 for purposes of clarity.
After data has been processed by the front-end processor, the processed data is supplied to the-base band processor 106. In spread spectrum systems, the base band processor has to be able to identify, despread, and decode the data. Despreading (i.e., multiplication of the process data by the same binary spreading waveform as was used to spread the data at the transmitter) and removal of the carrier modulation restore the original baseband data waveform.
In practice, multiple copies of the same signal are typically received at communications device 100 within a short time of each other. These copies, which are sometimes called multipath components arise because the signals take different paths of different length from the transmitter antenna to the receiver antenna. In the case of a CDMA system, it is feasible and advantageous to despread and decode several of the multipath components, realign them so that they are also in phase and combine them to produce a stronger signal. To do this, the base band processor in a CDMA system typically takes the form of a rake receiver that has several fingers, each one of which is a receiver that despreads and decodes one of the multipath components. General information about rake receivers can be found at pages 972–982 of J. S. Lee, L. E. Miller, CDMA Systems Engineering Handbook (Artech House 1998).
Generally, received data is sampled at a rate known as the chipping rate. In the IS-95 and 3 GPP CDMA standards, the chipping rates are 1.2288 MHz and 3.84 MHz, respectively, which correspond to sampling periods of 0.814 microseconds and 0.2604 microseconds. The sampling period is known as a chip.
Data is usually processed in pairs, commonly referred to as “In-Phase” (I) and “Quadrature” (Q) data. A rake finger is supplied with three samples, typically labeled Early, On-Time, and Late, where each sample includes a pair of data (i.e., I and Q data). The On-Time sample contains the data to be decoded. The Early and Late samples are used in tracking tools to ensure that the On-Time sample represents the center of the chip.
Two examples of chipping rate are 1.2288 MHz and 3.84 MHz which correspond to sampling periods of 0.814 microseconds and 0.2604 microseconds. These are used in IS95 and 3GPP CDMA systems, respectively.
Data arriving at the base band processor 106 is typically over-sampled at an over-sampling rate (e.g., 4× or 8× over-sampling rate) measured with respect to the chipping rate. For example: if the chipping rate is 1.2288 MHz and the over-sampling rate is 4× then the samples will be at 4.9152 MHz. Directly passing received over-sampled data to the base band processor 106 is generally inefficient because the sampling rate of the data may not be at the optimal rate for processing by the base band processor 106 rate. Thus, some type of front-end storage is required. Because the over-sampling rate is typically very fast, such front-end storage is likely to be very expensive, and more memory storage capacity is required as the sampling rate increases. Thus, a need arises for apparatus and methods that provide fast storage of raw data at the front end of the baseband processor without requiring excessive memory space.
Further, data is typically despread by multiple rake fingers. A need arises for apparatus and methods that allow reuse of multiple rake fingers to conserve physical silicon space and allow the multiple rake fingers to access the data storage substantially concurrently to achieve faster processing.